A diamond coated tool having a structure in which a material such as cemented carbide serves as a base material and the base material has a surface coated with a polycrystalline diamond layer, has been developed since long ago.
A diamond coated tool in which a base material has a surface coated with a diamond layer has a rigid surface, and is therefore capable of processing difficult-to-cut materials, such as fiber reinforced plastics (FRP) and the like, which have been considered to be difficult to process with a tool.
In applying a diamond coated tool to the difficult-to-cut materials described above, one of the determinants of the tool's life is adhesion between a base material and a diamond layer. That is, a diamond coated tool has a problem of being prone to exfoliation randomly occurring in cutting at an interface between a base material and a diamond layer, which results in a degraded grade of a cut material processed with a cutting tool after exfoliation and an unstable cutting tool life.
It has been conventionally attempted to increase adhesion between a base material and a diamond layer. For example, Japanese Patent Laying-Open No. 04-263075 (hereinafter referred to as “Patent Document 1”) proposes a diamond coated tool in which a base material having a surface on which fine asperities are formed is used and a diamond layer is formed on the base material.
As shown in Patent Document 1, by forming a diamond layer on a base material having a surface with asperities, an anchor effect occurs between the base material and the diamond layer. This anchor effect can increase adhesion between the base material and the diamond layer. Such a diamond coated tool is resistant to exfoliation in the early stage of cutting between the base material and the diamond layer.
The diamond coated tool of Patent Document 1, however, experiences high frequency of exfoliation at an interface between the base material and the diamond layer when cutting is continued for a long time, and the problem of exfoliation between the base material and the diamond layer has not been fully solved. Further, Japanese Patent Laying-Open No. 2002-079406 (hereinafter referred to as “Patent Document 2”) also discloses a technology to form asperities on a surface of a base material, as in Patent Document 1. The technology, however, does not show any remarkable improvement in prolongation of the life of a diamond coated tool.
Meanwhile, exfoliation between a base material and a diamond layer is believed to be caused by a difference between a coefficient of thermal expansion of the base material and a coefficient of thermal expansion of the diamond layer. That is, it is presumed that when a diamond coated tool reaches high temperatures due to heat in cutting, compressive or tensile residual stress is exerted on a diamond layer in the vicinity of an interface with the base material, thereby causing exfoliation to occur between the base material and the diamond layer.
For this reason, Japanese Patent Laying-Open No. 11-058106 (hereinafter referred to as “Patent Document 3”) takes an approach of relaxing residual stress occurring in a diamond layer when a diamond coated tool reaches high temperatures to achieve increased adhesion between a base material and the diamond layer. Specifically, adhesion between a base material and a diamond layer is increased by controlling the coefficient of thermal expansion and a material of a base material, the thickness of a diamond layer, and the like.